Not applicable
1. Field of the Invention
The present invention relates to the manufacture of aircraft, space vehicles and the like wherein panels are connected using friction stir welding. More particularly, the present invention relates to the construction of aircraft, space vehicles and the like wherein an improved method enables stringer stiffened panels to be joined with friction stir lap welds, in replacement of the traditional riveting practice.
2. General Background of the Invention
Friction stir welding (FSW) is a solid state joining process developed by The Welding Institute (TWI), Cambridge, England and described in U.S. Pat. No. 5,460,317, incorporated herein by reference. Compared with traditional fusion welding processes, it offers simplified processing, improved mechanical properties, diminished weld defect formation, equivalent corrosion resistance, and reduced distortion, shrinkage, and residual stresses. Using conventional milling equipment with a backside anvil support, a non-consumable, cylindrical pin tool is rotated and plunged into the butt or lap joint of the material to be welded. Pin tools are specifically designed for a given alloy and gauge. Also incorporated herein by reference are U.S. Pat. No. 5,718,366 and all references disclosed therein. The following additional references of possible interest are incorporated herein by reference: U.S. Pat. Nos. 3,853,258, 3,495,321, 3,234,643, 4,087,038, 3,973,715, 3,848,389; British Patent Specification No. 575,556; SU Patent No. 660,801; and German Patent No. 447,084. Publications that discuss friction stir welding include xe2x80x9cNew Process to Cut Underwater Repair Costsxe2x80x9d, TWI Connect, No. 29, January 1992; xe2x80x9cInnovator""s Notebookxe2x80x9d, Eureka Transfer Technology, October 1991, p. 13; xe2x80x9cRepairing Welds With Friction-Bonded Plugsxe2x80x9d, NASA Tech. Briefs, September 1996, p. 95; xe2x80x9cRepairing Welds With Friction-Bonded Plugsxe2x80x9d, Technical Support Package, NASA Tech. Briefs, MFS-30102; xe2x80x9c2195 Aluminum-Copper-Lithium Friction Plug Welding Developmentxe2x80x9d, AeroMat ""97 Abstract; xe2x80x9cWelding, Brazing and Solderingxe2x80x9d, Friction welding section: xe2x80x9cJoint Designxe2x80x9d, xe2x80x9cConical Jointsxe2x80x9d, Metals Handbook: Ninth Edition Vol. 6, p. 726. A publication authored by applicants is entitled xe2x80x9cFriction Stir Welding as a Rivet Replacement Technologyxe2x80x9d; Brian Dracup and William Arbegast; SAE Aerofast Conference, Oct. 5, 1999.
The present invention provides an improved method of constructing structures such as aircraft using friction stir welding, to thereby replace the traditional riveting practice of previously unweldable aluminum alloys. Possible applications include the intertank of the External Tank of the Space Shuttle and airplane manufacturing.
Friction Stir Welding is a Solid State joining process that now allows the welding of previously unweldable aluminum alloys. Traditionally, these aluminum alloys have found use only in mechanically joined structures such as in aircraft and space vehicles. The present invention provides a method of joining overlapped panels using friction stir welding, replacing the traditional riveting practice The method of the present invention is a viable, and cost reducing alternative to aluminum riveted structures.
The present invention features a non-consumable friction stir weld pin tool (see FIGS. 1-4) that is preferably constructed of H13 tool steel. The tool is rotated, plunged, and traversed along the stringer flanges of a stringer-skin panel to produce a friction stir lap weld. The tool is preferably tilted at an angle of about 2xc2xd degrees.
As the pin tool initially plunges into the weld jointline, the material is frictionally heated and plasticized at a temperature below that of the alloy""s melting temperature and typically within the material""s forging temperature range. When the metal becomes sufficiently soft and plastic, and the appropriate penetration depth has been reached, the tool is traversed along the weld line. As the tool is traversing, metal flows to the back of the pin tool where it is extruded/forged behind the tool. It then consolidates and cools under hydrostatic pressure conditions [2-8].
Unlike fusion welding processes in which there are numerous inputs to the welding schedule, friction stir welding requires only three: spindle speed (RPM), travel speed (IPM), and the penetration depth of the tool in the material (heel plunge or penetration ligament). Penetration depth can be monitored either through load control or displacement [9].
The present invention thus discloses a method and apparatus that uses a friction stir welding tool that fully penetrates through the top sheet and partially penetrates into the bottom sheet. The material around the pin tool is frictionally heated, plasticized, and extruded/forged to the back of the pin where it consolidates and cools under hydrostatic pressure conditions.
Friction stir lap welding stringer-skin panels will eliminate the inter-rivet buckling commonly seen on mechanically joined structures and consequently, increase the buckling strength of the vehicle. In addition, the present invention enables simpler processing as compared with the traditional riveting practice by replacing any touch labor required with an automated process. Eliminating the rivets and other associated parts will also reduce quality control and material handling issues. Consequently, friction stir welding will increase production build rates and reduce production costs. Overall vehicle weight will also be decreased by eliminating the rivets and their associated parts.